LDX Status Report
January 18, 2005
On Dec. 9-10, 2004, experiments were run on LDX in order to
investigate the relationship between the outer-boundary shape
and the ability of the dipole configuration to confine high-beta
plasma. The run plan was built around gradually increasing the
current in a large, external Helmholtz coil to form an "x-point"
(or toroidal field null) at different radii and limit the plasma
size. These experiments resulted in new and interesting data. In
particular, the kinetic interchange instability (driven by
energetic electron pressure) was identified, and a relationship
between the plasma compressibility and the nonlinear transport
induced by instability was observed.
While most of the experiments were conducted with "positive"
Helomholtz current, seven discharges were studied as the
reversed Helomholtz current was gradually increased up to -1.5
kA. On the eighth shot, the Helmholtz coil was energized to
-1.75 kA, and the coil tilted while attached to its supports. At
a 90-deg tilt, the coil struck the lifting tube and bent it. A
4-cm long dent a millimeter or so deep was made on the surface
of the floating coil cryostat. As described below, the floating
coil cryostat appears to be unharmed by the event, and repair of
the bent launcher system is underway.
FLOATING COIL EVALUATION AND CYROGENIC UPGRADES
The cryogenic performance of the floating coil has been
evaluated with several tests, and it appears to be unaffected by
the tilting event.
The first test consisted of cooling the floating-coil by liquid
nitrogen in it's usual location in the LDX charging station.
When the LN2 supply was interrupted, the warming rate was
observed to correspond to previous warming rates. Secondly, the
floating-coil was cooled by liquid helium. After the helium
vessel reached liquid helium temperature, the liquid helium
supply was interrupted, and, again, the warming rates were
observed to be similar to previous warming rates. Both cooling
and warming tests confirmed that no degradation had occurred in
the floating-coil thermal performance.
During this period of evaluation, we made some modifications to
the system to improve reliability and safety during system
operation. Guides were installed at the charging station ports
at the inlet and outlet of the transfer lines so as to provide
better alignments when the helium transfer bayonets are inserted
into the floating-coil ports. It is expected that a better
alignment will increase lifetime of the bayonet seals.
We measured the vacuum in the floating-coil vacuum space, which
had been sealed for about 6 months, and determined that it was
15 Torr. This indicates that the pressure in the cryostat had
increased to this value from 10(-5) Torr due to out-gassing of
the fiberglass shield and the MLI in the vacuum space during
this period of operation. We will obtain improved cryogenic
operation by vacuum pumping the floating-coil before each run
period using a port that had been designed for this purpose. To
allow pumping with LDX under vacuum, the removal device used to
remove the lower floating-coil pump-out plug was modified and
In addition to the existing vacuum-pressure gauges, two
convection vacuum transducers were installed into the guard tubes
at the entrance and exit of the cooling circuit to permit
control room monitoring of the pressure in the guard tube.
Additional valves have been installed into the liquid nitrogen
cooling system of the charging coil to provide more reliable and
safe operation and to reduce consumption of liquid nitrogen to
the radiation shield.
The lifting fixture repairs consist of replacement of parts,
straightening the lifting tube, and improving the launcher
pneumatic control console. The three lifting rod pieces (or
"spokes") have been purchased and are now being plasma
spray-coated prior to installation. New tabs have been
manufactured and will be welded onto the support "crown" this
week. The lifting tube has been straightened. The boron-nitride
lifting fixture insulator has been ordered. Finally, the new
launcher control console has been designed.